The present invention relates to sheet feeder systems and more particularly to those systems wherein the stack of sheets is continuously supplied to the distributor so that the arrangement can operate continuously. The present invention is particularly advantageous when utilized for the control of the transport of large flat sheets.
It is particularly important in such arrangements that, after the introduction of a new stack into the system, the stack is moved rapidly into the operative vicinity of the distributor and that the distributor can operate continuously to separate the sheets from the stack and to transport the individual sheets towards the next processing station.
In known arrangements of the above-described type, pawl drive mechanisms are utilized and the stack is transported step-by-step in synchronism with the operation of the remainder of the machine. The drive mechanism is subject to such great wear at higher machine outputs that breakage can result. Further, with this type of drive, only constant lifting intervals can be realized and thus, depending upon the mechanical layout, only a limited number of sheets can be processed per interval.
In order to overcome this disadvantage, further known systems replace the step-by-step operating mechanical drive by electric motors which also operate step-by-step. In such motors it is possible to create varying intervals by the use of timing elements. However, these electromotor drives also have disadvantages. First of all, under high output conditions and particularly when processing a sheet load approaching the maximum possible for the set interval, the motors are often overloaded which causes the windings to burn out. This of course limits the maximum possible output. Further, the systems have the disadvantage that the cycle is initiated by a single signal furnished by a microswitch or similar means. Thus an automatic control under changing conditions is impossible and the contact wear limits the reliability of the arrangement and severely limits the maximum possible switching frequency.
The above-described systems can be found in patents BRD-AS1,102,770 and 1,118,812 of the German Democratic Republic.
Further, in the publication BRD-AS 1,217,406, transport systems for sheet feeder arrangements are known which utilize either contactless or contact sensors to furnish the signals for causing a change in the position of the stack. This can result in a continuous transport system. However, these systems have various disadvantages when large size sheets are being processed. First, the density of the stack at different places differs and the top surface of the stack is also quite variable.
For this reason contactless sensors as, for example, described in BRD-AS 1,189,088, have heretofore only been used to control the movement of the support means for the stack, such as a table, in the downward direction, that is away from the distributor means in order to receive a new stack. A further disadvantage of the contactless sensor systems is that the tight tolerances which are required in defining the operative vicinity to the distributor means cannot be maintained by such sensors. However, the system using contact-type sensors which allow the maintenance of these tolerances have other disadvantages. These systems again operate mainly with microswitches which, as previously mentioned, results in high contact wear and therefore a limiting of the possible switching frequency, a lack of reliability and a relatively short lifetime for the system.
Further arrangements for positioning the stack are shown, for example in BRD-AS 1,263,028, wherein the drive is continuously adjustable and wherein the position of the individual sheet is sensed in a contactless manner either photoelectrically or inductively, and control members for regulating the transport speed of the stack are operated in response to these sensors. The control members may be control transformers, resistors, magnetic amplifiers or thyritron amplifiers or even controllable silicon rectifyers. For universally applicable stack feeds for flat sheets, which may reach relatively large sizes, these control members are not usable since they are operative only for low power outputs and have a poor efficiency when high power outputs are required. Specifically for a constant torque, only a small region of adjustment is available. However, particularly for sheet feeder arrangements of the above-described type and utilizing broad sheets varying in thickness from paper to cardboard, a large adjustment region is required.